1. Technical Field
The present invention relates to optical devices, and in particular, optical waveguides that are coupled with heaters.
2. Art Background
Optical fiber communication systems are becoming more prevalent. In addition to the optical fiber itself, optical fiber communication systems use a wide variety of optical devices for receiving, transmitting, and using optical signals. One type of integrated optical device is a silica optical circuit formed on silicon substrates. The basic structure of such device is described in Henry, C. H., et al., xe2x80x9cSilica-based optical integrated circuits,xe2x80x9d IEE Proc.-Optoelectron, Vol. 143, No. 5, pp. 263-280 (1996).
In certain optical devices such as thermo-optic switches, heaters are used to effect a change in the refractive index of the device. The change in refractive index of the device changes the phase of the signal transmitted through the device. Using a heater to effect a controlled change in the refractive index of the optical device is a way to effect a controlled change in the phase of the transmitted optical signal. Inducing a controlled phase change in an optical signal is useful in a variety of contexts. In some devices a controlled phase change is induced to tune the optical device. In other devices, a controlled phase change is used as a switching mechanism.
Examples of thermo-optic switches include the Nxc3x97N optical switch and the add/drop multiplexer. In thermo-optic switches, a thin film heater is formed on the optical device, which is typically a planar waveguide. The planar waveguide consists of a first cladding layer formed on a substrate. The core is formed on the first cladding layer. A second cladding layer is then formed over the core. A thin film heater is then formed on the second cladding layer. The thin film heater is used to effect a desired change in the refractive index of the core and claddings for device operation.
To produce the desired phase change in the device, the temperature of the waveguide is increased. However, the waveguide is in intimate contact with the silicon substrate, which is a good thermal conductor. Consequently, unless the silicon substrate is thermally isolated from the waveguide, it is difficult to effect a local change in waveguide temperature and the power consumed by the heater will be high.
Many devices, such as Mach-Zehnder waveguide interference devices, have multiple branches or arms that must be independently heated to effect the desired device operation. For example, in the two-arm Mach-Zehnder devices, one arm is heated and the other arm is not to produce a desired differential phase change between the signals from each arm. Since silicon is a good thermal conductor, silicon facilitates temperature equilibration between the heated arm and the unheated arm, which is undesirable. Consequently, there have been attempts to thermally isolate the arms of a silicon-based planar Mach-Zehnder device from each other.
In one approach, trenches are etched out under the waveguides to thermally isolate the waveguides from the substrate. However, the trenches increase the amount of time for the device to cool and thus the amount of time it takes for the device to switch back to its unswitched state (the switched state being the heated state). Another disadvantage of the trench approach is the complexity of manufacturing involved in extra steps.
Accordingly, ways to control the heat flow from a heated waveguide that is conducted by the substrate are sought.
The present invention is directed to an optical device in which heat is used to thermally induce a desired change in the refractive index of the device. The device is a waveguide that is formed on a region of porous silicon that is formed in a silicon substrate. The porous silicon region has a thermal conductivity that is less than the heat conductivity of silicon oxide. Consequently, less heat is conducted from a heated waveguide formed on a porous silicon region that would otherwise be conducted if the heated waveguide were formed on either silicon or silicon oxide.
The optical device is fabricated by first forming at least one region of porous silicon in a silicon substrate. Expedients for forming regions of porous silicon in a substrate are well known to one skilled in the art. It is advantageous if the porous silicon is formed using an electrolytic process. In the electrolytic process, silicon is anodized in an electrolytic solution. Such a technique is described in Unagmi, T., et al., xe2x80x9cAn Isolation Technique Using Oxidized Porous Silicon,xe2x80x9d Semiconductor Technologies, Vol. 8, Chap. 11, pp. 139-154 (OHMSHAT and North Holland Publishing Company 1983) which is hereby incorporated by reference.
The silicon substrate is anodized selectively to form porous silicon regions therein. The substrate is selectively anodized by forming a mask on the silicon substrate prior to anodization. The mask has at least one opening therein. The underlying silicon substrate surface is exposed through such openings.
Similarly the porosity of the porous silicon region is also largely a matte of design choice. The porosity of the material is controlled by the doping level of the substrate (e.g. silicon wafer) and the anodization condition used to form the porous areas. Anodization conditions such as an applied voltage and the associated current density as well as the concentration and pH values of the HF solutions are selected to obtain the desired porosity. The range of appropriate porosities is mainly determined by two factors: low heat conductivity and mechanical stability. The porous silicon must be porous enough to have heat conductivity lower than that of silicon oxide (1.4 W/m K). In this regard, it is advantageous if the porosity is at least about 50 volume percent. In principle, the higher the porosity is, the lower is the heat conductivity (G. Gesele, G, et al., J. Phys. D: Appl. Phys., Vol. 30, pp. 2911-2916 (1997). Therefore, higher porosity is advantageous because it provides lower heat conductivity. In certain embodiments, the porous silicon need only withstand the rigors of subsequent processing. In these embodiments, porosities up to about ninety-five percent are suitable. In other embodiments where the structural demands on the porous silicon are more rigorous, porosities of up to ninety percent are contemplated. This is because thick (i.e. greater than about 5 xcexcm) porous layers are brittle and do not provide the required support for the optical device formed thereon.
The dimensions of the porous silicon region are largely a matter of design choice. The depth and size of a porous silicon region depends upon the size of the waveguide subsequently formed on the region and the amount of heat generated by the heater on such device. Since the objective is to stem the heat conducted from the heated waveguide into the substrate, the larger the amount of heat generated by the waveguide, the greater the resistance to thermal conductivity that must be provided by the porous silicon region. As previously noted, one way to increase the porous silicon""s resistance to thermal conductivity is to increase its porosity. Another way is to increase the distance heat must travel through the porous silicon region (i.e., the thickness of the porous silicon region) to the substrate.
After the porous silicon region is formed on the substrate, the optical device is completed. Examples of devices that would benefit from being formed on porous silicon include Wavelength Add-Drop (WAD) for Dense Wavelength Division Multiplexing (DWDM) systems, dynamic wavelength equalizers for chromatic equalization in Er-doped fiber amplifier systems; Mach-Zehnder (MZ) based switches, tunable filters, and Y-branch switches. The thickness and porosity of the porous silicon region are selected to balance the efficiency of the device (i.e., the amount of heat needed to switch) and the speed of the device. The better the insulation of the element from the silicon substrate (which functions as a heat sink) the longer it takes for the heat to dissipate from the heated portion of the device, and the lower the frequency of the operation.
Furthermore, the processes used to form the waveguide on the porous silicon region must be compatible with the porous silicon process. This means that the process for forming the claddings and cores of the devices on the porous silicon can not require conditions that would damage the porous silicon region. For examples, the temperature range in which the claddings and cores are deposited on the porous silicon region cannot include temperatures that will cause the porous layer to disintegrate. Forming the cores and claddings on a porous silicon regions using a standard low-pressure chemical vapor deposition (LPCVD) technique does not unacceptably damage the porous silicon.